The warning on Friday comes a day after the death of a 22-year-old Indonesian chicken seller, which local tests showed had been infected with the H5N1 bird flu virus.
If confirmed, it would bring to 15 the number of people known to have died from bird flu in Indonesia. Five other people have survived infection from a virus that scientists fear could trigger a global pandemic in humans.
Traditional wet markets are common throughout populous Indonesia and Alexander von Hildebrand, the WHO's regional adviser for environmental health, said vendors often conducted business on dirty ground, placing everyone at risk of infection.
Many vendors are clueless about the H5N1 virus surviving in chicken droppings for days, he said.
"The exposure to poultry by market stall owners, slaughterers, poultry workers and the customer in the wet marketplace demonstrated that awareness of avian influenza, transmission routes and methods of preventing transmission is limited," he said.
Keeping ducks and chickens adjacent also presented problems.
"Some vendors are keeping chickens very close to ducks which can be a problem because ducks do not show the disease but can carry it and transmit it," he said.
Sanitation in many traditional
markets is poor
If an infected bird was present, then many people risked being exposed to the blood and secretions, he said. "Re-zoning is necessary to limit the potential public exposure."
Millions of Indonesians shop at traditional markets where fruit, vegetables and meat are often sold on the ground in the midst of slush and dirt.
Sanitation in many traditional markets is poor, with dirty or drainage water used to wash produce and stalls.
The WHO has already called for preventive measures, including limited contact between humans and poultry in markets, as well as better access to water and improved waste management.
Increasing the risks is that H5N1 is endemic in poultry in parts of Indonesia and in addition to unsanitary markets, many chickens and ducks live closely among people on small farms or even in cities and towns. This raises the chances of more humans becoming infected.
The government says the highly pathogenic strain of H5N1 has been detected in birds in two-thirds of the provinces. A further complication is that Indonesia, with 220 million people, comprises about 17,000 islands, making surveillance and control measures more difficult than many other countries.
"Some vendors are keeping chickens very close to ducks which can be a problem because ducks do not show the disease but can carry it and transmit it"
Alexander von Hildebrand,
WHO Regional Director
H5N1 is not known to pass easily between humans at the moment, but experts fear it could develop that ability and set off a global pandemic that might kill millions of people.
In total, the virus has killed at least 83 people in six countries since late 2003. Millions of poultry have either died or been culled to try to stop the virus spreading.
But Indonesia has not carried out the mass culling of some countries, in part because it cannot afford to compensate farmers for destroyed birds.